The objective of this research is to study the effects of environmental deprivation and malnutrition early in life on mental development and to establish to what extent it is possible to prevent intellectual retardation by food supplementation and/or psychological stimulation. Malnutrition is known to be associated with impaired psychological function. This relationship is confounded, however by health and social factors that covary with nutrition and may themselves affect psychological development. To investigate these complex relationships we have undertaken a prospective multidisciplinary intervention study of children at risk of malnutrition starting at six months of gestation. Families are assigned at random to six study groups: A) No supplementary feeding; B) Supplementary feeding from six months to three years of age; C) Supplementary feeding from six months of gestation to six months of age; D) Supplementary feeding from six months of gestation to three years of age; A1) A home intervention program designed to increase psychological stimulation of the child but no supplementary feeding; D1) Supplementary feeding from six months of gestation to three years of age plus psychological stimulation. All groups are given a uniform obstetrical and pediatric health-care program. Subjects are observed longitudinally for the duration of intervention and for three years thereafter, through repeated measurement of nutritional status, intellectual functioning and a number of concomitant social and health variables. Groups A, A1, D, and D1 constitute a two-component factorial experiment that allows assessment of simple and interactive effects of supplementation and stimulation. Groups A, B, C, and D permit study of the effects of supplementation at different age levels. Group A by itself constitutes an ecological study of malnutrition and social deprivation since the only intervention it receives is free medical care and measurements. Results to date show significant effects of supplementation on birth weight, physical growth, maturation and locomotor development while early stimulation affected cognitive competence. Longitudinal follow-up for three years beyond the intervention will assess the permanence of the effects and permit detailed study of preschool cognitive development in a deprived social setting.